An active common mode filter for a PWM inverter is known and is described in detail in application Ser. No. 09/816,590, referred to above. The active common mode filter of the above application includes an amplifier and current driver circuit located in the d-c link between the rectifier and the inverter of a pulse width modulated driving circuit.
For that filter, as shown in FIG. 1, an a-c line input having terminals L1, L2 (for a single phase) and ground (GND) is connected to a full wave bridge connected rectifier 10 which is coupled by a d-c link P, N to a 3 phase PWM inverter 12 (which may employ thyristors or IGBTs in each leg, with suitable controls, not shown). Invertor 12 then drives the a-c motor 13. The grounded case of motor 13 is connected to GND and to coupling capacitor 14. An active filter 11 includes a ground current sense transformer 15 which produces an output to op-amp 16. This, in turn, controls the gates of transistors Q1 and Q2 of current driver circuit 17 coupled across the d-c link to switch the transistors between a non-conducting state and a conducting state in a linear region.
Amplifier 16 receives an input from a secondary of the current transformer 15, which has two primary coils coupled in the two legs of the d-c link. The current transformer provides an output on its secondary proportional to the common mode current which equals the leakage or noise current in the ground line G from the motor. This noise current is generated across the internal capacitance between the motor windings and the motor case, and flows in the ground line G. This noise current can be substantial and can also generate significant EMI.
The circuit of FIG. 1 is designed to cancel the noise current by replicating the current through the isolating capacitor 14, allowing the current to travel back to the buses of the d-c link. This substantially eliminates the ground current on the GND line back to the a-c network and therefore the common mode noise current present on the a-c network. Thus, the filter 11 reproduces the common mode current that flows from the d-c link into the inverter, creating circulating paths for the common mode current via the ground wire from the motor, the coupling capacitor 14 and back to the d-c link, bypassing the common mode current from the a-c input lines.
The circuit in FIG. 1 may not always be practically convenient, because it involves xe2x80x9cbreaking intoxe2x80x9d the d-c link to insert the active filter 11 and the current sensing transformer 15.
In some applications, it would be preferable to insert the common mode filter 11 in the input a-c line that feeds the rectifier 10 of the PWM driving circuit. This would conform to the usual connection point of a normal passive common mode filter, which is commonly connected in the a-c line. If the active common mode filter could also be placed in the a-c line, it would be a direct substitute for the normal passive common mode filter, and would not require modifying the PWM drive itself.
It is accordingly an object of the present invention to provide an active common mode filter which can be constructed as a form of a-c filter which can be added on to an inverter without modifying the inverter circuit.
It is further an object of the invention to provide such an active common mode filter which can be utilized both in a feed forward or feed back architecture. In a traditional feed back architecture for a common mode filter circuit, the common mode noise is sensed after the cancellation point and the canceling signal is injected by an amplifier having high gain and fed back to the point of noise cancellation. In contrast, in a feed forward architecture, the common mode noise is sensed before the cancellation point and the canceling signal is injected by a gain of one amplifier and fed forward to the noise cancellation point.
It is still a further object of the invention to provide an active common mode filter circuit which does not require modification of a system into which it is connected.
It is yet still a further object of the present invention to provide an active common mode filter circuit which can directly replace a passive a-c line common mode filter.
The above and other objects of the present invention are achieved by an active filter for reducing common mode current in a pulse width modulated driving circuit for driving a load from a-c lines of an a-c source, wherein the driving circuit includes a rectifier connectable to the a-c lines and producing a rectified output voltage connected to a d-c bus, a PWM inverter having input terminals connected to the d-c bus and having a controlled a-c output for driving the load, the active filter comprising a current sensor for sensing the common mode current in the driving circuit, a current driver circuit comprising first and second transistors, each having first and second main electrodes and a control electrode, the transistors being coupled in a series circuit to feed a single output node between them, the transistors being controlled such that they are alternately switched on and off by control of their control electrodes, their control electrodes being connected to an output from the current sensor, the output node between the transistors being coupled to an isolating capacitor, the isolating capacitor being coupled to a ground line coupled to the load, the two transistors providing alternate paths for current in the ground line to flow to the a-c lines of the a-c source thereby to reduce the common mode current flowing from the a-c source by providing a circulating path for the current the active filter being connected in the a-c lines coupled to the pulse width modulated driving circuit.
The above and other objects are also achieved by an active filter for reducing common mode current in a driving circuit for driving a load from a-c lines of an a-c source, wherein the driving circuit provides a controlled a-c output for driving the load and the load has a ground line coupled to the a-c source, the active filter comprising a current sensor for sensing the common mode current in the driving circuit, an active filter circuit comprising a transistor current driver circuit controlled by the current sensor such that it provides a circulating path for a ground current in the ground line, the current driver circuit being coupled to an isolating capacitor, the isolating capacitor being coupled to the ground line coupled to the load, the current driver circuit enabling the current in the ground line to flow to respective ones of the a-c lines of the a-c source thereby to reduce the ground current flowing in the ground line, the active filter circuit being connected in the a-c lines coupled to the driving circuit.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.